1. Field of the Invention
The present invention relates to a numerical control device, and in particular, to a type of numerical control device (hereinafter referred to as a CNC device) incorporating a small-sized computer.
2. Description of the Related Art
A display/operation unit in a machine tool including a CNC device is mounted on a machine operation panel disposed at the front of the machine, for the convenience of an operator's monitoring and operation. But, a CNC controller and a servo amplifier are mounted on a heavy-electrical panel of the machine, since they can not be located on any specific place and also their external dimensions are relatively large. Thus, the CNC controller is connected to the display/operation unit through a cable. A CRT display or a liquid crystal display (hereinafter referred to as LCD) is used as a display for the CNC display/operation unit. A disadvantage of the CRT display is that it occupies a large installation space due to its large depth, whereas the LCD can be made more compact due to its smaller depth.
In the case where a CRT display is used, it does not matter if a CNC controller and a CRT display are connected directly to each other by means of a long cable. By contrast, in the case of a LCD, it is impossible to directly connect a CNC controller and a LCD with a long cable, and a synchronizing circuit with respect to a liquid crystal control signal is indispensable, thereby giving rise to a problem of an increase in cost.
Signals for controlling a CRT display by a CNC controller may be a horizontal synchronizing signal, a vertical synchronizing signal, red, green and blue color signals, etc. The color signals are analog signals, and a brightness of the color signal is determined by a level (voltage level) of each color data. Thus, a slight change of the level during signal transmission would not affect the brightness of color signal that much. Further, if time lag of the color signal happens, as far as it is small, change in color is negligible and the color difference is hardly recognized by the human eye. Therefore, the color signal has a large allowable range with respect to the change in its level and time lag. Thus, in the case where the CNC controller and the CRT display are connected through a cable for transmitting an image control signal such as a color signal, if the length of the cable is within the dimensions of the machine (the length of such a cable is usually about 10 m or so), the change in the level or a time lag of the color signal caused during a transmission process through the cable does not bring about a change in brightness or color as much as human is capable of recognizing with his eyes. Thus, it is possible to directly connect the CNC controller with the CRT display through a cable.
Meanwhile, in the case of the LCD, a standard type LCD is capable of displaying 512 colors to 260 thousand colors. Such color signal, as it is a digital signal, requires signal lines of 3.times.3 (512 colors) to 6.times.3 (260 thousand colors), and further signal lines for liquid crystal control for horizontal and vertical synchronizing signals and so on. Since the color signal is a digital signal as described above, if an error occurs at a higher-order bit of the color signal (for example, if the bit is "0" when it has to be "1" or if the bit is "1" when it has to be "0"), brightness undergoes a change by 50%.
Moreover, in the LCD, a dot matrix is used for display, and a display ranging from 640.times.480 dots to 800.times.600 dots is currently possible. In order to display on a screen an image having no flicker, there is a need of displaying an image on a screen at a cycle of 60 to 80 times per second. For this reason, a transmission must be made with a clock of about 25 MHz (=(640+.alpha.).times.(480+.alpha.).times.60) to 40 MHz (=(800+.alpha.).times.(600+.alpha.).times.80)(where, .alpha. is a positive integer). Further, as display is done line by line, a sampling clock of 25 MHz to 40 MHz synchronous with a color signal is required in order to accurately fetch color data. In this case, however, there occurs a dispersion in a time lag of a signal due to the characteristics of drive and receiving circuits for transmitting and a cable. Thus, it is very difficult to make a long-distance transmission (about 10 m) without using the clock of 25 MHz or more and without causing any time lag. If there is any difference in the time lag between the clock and the color data signal, color data cannot be sampled. Moreover, if there is a dispersion in the time lag of the color data signal, the color data transmitted as the same dots may be sampled as different data, and thus is displayed as other dots, with the result that a different color will be displayed.
Thus, the connection of the CNC controller and the LCD with a long cable has to be avoided. Therefore, the LCD side has to be provided with a synchronizing circuit for the liquid crystal control signal such as color signal, etc.
In order to reduce the cost of a machine tool including a CNC device, it is important not only to reduce the cost of a machine tool itself but also to reduce the costs of other mechanical and electrical equipment which account for a substantial part of a CNC machine tool cost. Although it is possible to reduce the cost by mass production, but, since machine tools will be used over 20 years to 30 years, the demand for the machine tools are not as large as that for the consumption goods, so that there is a limitation of the cost reduction by mass production. However, CNC devices for machine tools are designed to be applicable to various kinds of machine tools and thus compatible with one another, so that effect of the mass production can be expected to some degree. Thus, the cost reduction of the CNC device greatly contributes to the cost reduction of the machine tool.
In order to achieve the cost reduction, there has been proposed a method of employing a micro computer (hereinafter, referred to as a personal computer) which can be used for various purposes and is available at low cost by mass production so as to constitute a CNC device. However, a CNC device differs from a personal computer in working environment and functions to be required. For this reason, it is difficult for a personal computer to meet the specifications required for machine tools.
A CNC device for use in a factory is exposed to a considerably severe working environment as compared with environment such as that of an office or the like where the personal computer is used. The CNC device is supposed to be capable of meeting requirements of operating conditions such as an allowable temperature range of 0.degree. C. to 45.degree. C., the maximum humidity of 95% and the maximum vibration of 0.5 G, of providing against an instantaneous power interruption conforming to the IEC (International Electrotechnical Commission) standard, and of taking measures for avoiding influences of dust, metallic powder, water, cutting oil, etc. In contrast, in the case of a personal computer, the allowable operating temperature range is 0.degree. C. to 35.degree. C., and the measures against moisture, vibration, instantaneous interruption of power source, and measures for airtightness against dust, metallic powder, water, cutting oil, etc. are not available. Thus, a personal computer is largely different from a CNC device in structural adaptability to operating environment. Therefore, a CNC device requires a structure durable in severe working environment as compared with a personal computer.
For instance, in a CNC device, a floppy disk has a problem of not adapting to an operating environment of a factory in view of temperature, humidity, vibration, etc. Also, a hard disk is not capable of satisfying operational requirements of a CNC device in view of temperature, vibration or the like. For this reason, a hard disk is not basically used either, though it is used only for optional function. Instead of these storage media utilizing magnetism, the CNC device uses storage media such as a flash memory (electrically rewritable nonvolatile semiconductor memory) for storing software, and an SRAM (rewritable nonvolatile semiconductor memory) which stores NC machining programs, various parameters, backed up with a battery, for bearing a working environment requiring operational conditions such as those in a factory.
Moreover, a CNC device is required to have a high-speed data processing ability for controlling a servo motor for individual axis of a machine tool on real-time basis. However, a personal computer, employing a commercially available OS (Operating System), is ill fitted for real time operation, so that its ability can not be sufficiently exhibited. In order to meet the need of high-speed arithmetic operation, the present-day CNC device is provided with a plurality of processors such as a CNC main processor, which performs a function of interpreting an operation command to an NC and giving a movement command of each servo motor to a servo controller and a function of making a communication with an operator who operates machine tool through a keyboard and display, a PMC (Programmable Machine Controller)-dedicated processor, which executes transmission/reception of an ON/OFF signal with the machine tool to perform sequence control for machine tool, a DSP (Digital Signal Processor), which has been developed for signal processing in communication with high-speed data processor for servo control, thereby making available high-speed arithmetic operation for the control of a machine tool. However, since a personal computer is provided with only one processor, it is difficult to employ a personal computer in a control device which is required to perform real-time control of machine tools at a high speed.
Furthermore; a CNC device has to be added various optional functions to provide with a general-purpose ability so that it may be applicable to various machine tools ranging from single-function type to multi-function type and from standard performance type to high performance type. For example, various functions which need to be added to a CNC device are as follows: a loader control function for carrying out handling of workpiece in machining; a RISC (Reduced Instruction Set Computer) function for executing arithmetic operation for realizing high-speed machining with a high accuracy by previously reading a plurality of blocks of NC data in continuous micro blocks such as those of mold machining data to execute acceleration/deceleration control; a communicating function for executing data transmission with external computers through an Ethernet, RS-422, RS-232C, etc.; a customizing function for enabling a software which each machine manufacturer has originally developed for each machine tool independently of the processor of a CNC device. Thus, a CNC device must be constructed so that units and circuits for executing the foregoing functions can be connected therewith.
However, a personal computer is not so constructed as to permit an addition of various kinds of optional functions in machine tools which is required to have a high-speed arithmetic operation ability as described above. At present, in the personal computer, an ISA (Industrial Standard Architecture) bus as a bus for extending optional functions is included in specifications and is now a de facto standard of the manufactures. The ISA bus has a bus width of 16 bits, and the whole address space accessible through the bus is 16 MB, but it is not designed to permit each device connected to the ISA bus to operate as a master of the bus. Therefore, the ISA bus is not so constructed as to permit an addition of various kinds of optional functions for machine tools which is required to have a high-speed arithmetic operation capability.
As described in the foregoing, it is difficult to obtain functions and construction required for the current CNC device with the use of the personal computer. Thus, it is desirable to achieve a cost reduction of machine tools with use of the presently available CNC device. To achieve the cost reduction of machine tools, there is proposed a method of manufacturing a small-sized machine tool. However, in order to manufacture the small-sized machine tool, it is necessary to reduce the size of not only a machine operation panel but also the size of a CNC device itself. In particular, if a machine operation panel for a machine is made smaller in size and thinner in depth, then places which allow the machine operation panel to be installed will increase, thereby contributing to downsizing of the machine tool because of its good operational capability and small space occupied.